If renting the Olympic stadium to a football club rather than selling it to one is the excellent idea London's Mayor has been claiming it is since, well, Tuesday, why didn't he go for that option in the first place? It's a reasonable question, as Boris himself acknowledged when the Green Party's Darren Johnson put it to him at mayor's question time this morning.

"The answer is that in tough financial times it is always sensible to look at whether you can privatise," Boris replied in a suitably reasonable way. "Unload the whole thing on to the private sector. It wasn't an irrational thing to do." And yet, the mayor continued, "If you build a stadium with taxpayers' money you're obviously going to invoke state aid issues in the disposal of that stadium to a private sector party. There are inevitably going to be complications."

Obviously? Inevitably? If they were that obvious and inevitable, might they not have been anticipated some time ago? Leyton Orient chairman Barry Hearn spotted them pretty sharpish. So did that anonymous person (not Hearn himself, he says) who raised the "state aid" spectre with the European Commission and in so doing seems finally to have finished off the litigation-encrusted West Ham ownership deal.

Boris presented that deal's collapse as "a very sensible solution" for "ending the faffing around" caused by "a great number of legal anxieties" which included Spurs' dogged efforts to get the decision to award the stadium to the Hammers overturned. But hadn't Boris encouraged the north London club to migrate east from Tottenham in the first place asked Labour's Joanne McCartney, who represents the area.

It's not the first time this suggestion has been made. Boris's reply was the nearest to a "yes" that I've yet heard from him. "I said to them [Spurs] that if they were interested they should certainly bid," he said. "They were very keen to bid for it and I said that if they wanted to bid for the Olympic stadium I saw no harm in that. Indeed it was completely the right thing to do because it would have been absolutely mad to get into a situation in which we had no competition whatever."

That defence – which I anticipated here – is a respectable one, though it also confirms that Boris bears some responsibility, albeit indirect, for the "faffing" he is now congratulating himself on ending.

Yet his critics are luxuriating in the bubble bath of hindsight. They can afford to because Boris has limited scope for criticising those responsible for including a solemn pledge to retain an athletics track in the Olympics bid in the first place. He doesn't want to attack games organiser and fellow Tory Lord Coe, whom many regard as the prime suspect – imagine the fun the press would have with that – which means he can't really attack the mayor of the time Ken Livingstone either. The Labour group is unlikely to rush to his assistance.

This leaves the Conservatives' Andrew Boff as the sole beacon of shining consistency. He has long maintained that unless the athletics track goes the stadium is destined to turn pale, grow a trunk and feed itself from the public purse.

The latest potential black hole to form in the stadium's finances was not examined much on Wednesday, but it looks roughly the size of £50 million for converting the stadium into something smaller and football-friendlier after the games. And then there's the worry that even if West Ham move in as leaseholders their annual rent, reckoned to be about £2 million, won't cover annual costs, estimated to be around £3 million more.

Boris was, as ever, stoutly upbeat on this point, but he has an unhappy record for over-selling the potential of private investment in projects designed for London's public good. He once anticipated that the development costs of his new London bus would be met by the manufacturer. In the end they were met entirely by Transport for London (£7.8 million).

His transport manifesto posited a cycle hire scheme financed entirely by private sponsorship, yet in the end Barclays is paying for but a fraction of what has so far been a loss-maker.

On Wednesday he provided rather thin answers to questions from the Liberal Democrat's Mike Tuffrey about the financing of his Thames cable car project and the true value of the contribution by airline sponsor Emirates. Originally this venture too was to have been paid for without public money, yet £60 million has already been stumped up with no clear picture of how much of that sum might be recouped or when.

As for the Olympic stadium I'm warming to Barry Hearn's idea of digging down a bit further, lowering the athletics track and by so doing enabling the installation of retractable seats as they have in the Stade de France. I haven't a clue if the ruse is even practical, let alone what it would cost or who would pay. But it would make the stadium truly viable for both football and track and field. Why did no one think of that in the first place?